The invention pertains to the field of integrated bipolar transistor construction, and, more particularly, to the field of bipolar transistors which have a base area with reduced size and the concomitant reduction in the parasitic capacitances and resistance.
Workers in the art of integrated bipolar transistor design have long known that a reduced base area is desirable for several reasons. First transistors with reduced base area have less parasitic junction capacitance associated with the base-collector junction because there is less area of the junction and the parasitic capacitance of this junction is proportional to the area of the junction. Second, the smaller base area means the overall size of the transistor is reduced, and, therefore, more transistors may be put on a single die.
Smaller parasitic capacitances coupled to the base means faster switching operation for such transistors. Since computers typically employ millions of switching transistors and perform millions of operations involving switching by these transistor every second, faster switching transistors mean that more operations per second may be performed. This translates to faster data processing and more output per unit of time. Smaller transistors means more functions may be put on a single integrated circuit die. This translates into cost savings because fewer connections need be formed by soldering because fewer integrated circuits are used to accomplish any given function. Thus, smaller transistors means lower cost electronic products. Further, reliability of such products is increased because a connection made internally on an integrated circuit is far more reliable than a soldered connection made in the external world.
The problem with prior art bipolar transistors is that the base area can only be made as small as the minimum linewidth, D, permits. The minimum linewidth, as those skilled in the art appreciate, defines the size of the smallest geometric feature which may be formed on the surface of an integrated circuit for a given photolithography system. In the prior art, the base area surrounds the emitter region. The base area needs at least one electrical contact formed thereon, and since the minimum dimension on each side of the emitter region is one D. Since the design rules require a certain clearance around the emitter which the base contact may not encroach upon, and since the base contact must be at least one D wide, the minimum size of the base area may not be smaller than the above criteria permit. Thus, the performance levels of prior art transistors have been limited, and improvements, at least in part, have been linked to reductions in minimum obtainable linewidth.
Thus, a need has long existed for an improved bipolar transistor with a smaller base area than is possible with conventional construction.